1. Field of the Invention
The present invention relates to ADC (analog-to-digital converter).
2. Description of Related Art
Persons of ordinary skill in the art will understand terms and basic concepts related to microelectronics that are used in this disclosure, such as PMOS (p-channel metal-oxide semiconductor) transistor, “source degeneration,” “voltage,” “current,” “oscillation,” “voltage-controlled oscillator,” “ring oscillator,” “frequency,” “clock,” “phase,” “analog-to-digital converter,” and “digital-to-analog converter.” Terms and basic concepts like these are apparent from prior art documents, e.g. text books such as “Design of Analog CMOS Integrated Circuits” by Behzad Razavi, McGraw-Hill (ISBN 0-07-118839-8), and thus will not be explained in detail here.
A voltage-controlled oscillator (VCO) receives a voltage and outputs an oscillation signal, whose frequency (of oscillation) is determined by the voltage. An ideal transfer characteristic of a VCO is depicted in FIG. 1. As shown, the frequency (of oscillation) increases linearly as the voltage increases; when the voltage is Va, the frequency is fa; when the voltage is Vb, the frequency is fb. If the frequency of the oscillation signal can be detected, then the voltage can be inferred and thus detected. Sometimes, it is more convenient to detect the phase, instead of directly detecting the frequency, of the oscillation signal; in this case, the frequency can be indirectly detected by first detecting the phase and then performing a time derivative on the phase. As long as the frequency is detected and represented by a digital word, then the digital word is a digital representation of the voltage. In this manner, a VCO can be used to embody a digital-to-analog converter (ADC).
As depicted in FIG. 2, a VCO based ADC 200 comprises: a VCO 210 for receiving a voltage V and outputs an oscillation signal K; a phase-to-digital converter (PDC) 220 for receiving the oscillation signal K and outputting a phase sample P representing a phase of the oscillation signal K in accordance with a timing provided by a clock CLK; and a derivative operator (1-z-1) 230 for receiving the phase sample P and outputting a digital output D. In an embodiment, VCO 210 comprises: a voltage-to-current converter (V2I) 211 for converting the voltage V into a current I, and a current-controlled ring oscillator (ICRO) 212 for generating the oscillation signal K in accordance with the current I. Phase-to-digital converter (PDC) 220 comprises: a ring sampler 221 for outputting a digital sample S by sampling the oscillation signal K using the clock CLK, and a phase decoder 222 for decoding the digital sample S into the phase sample P. The frequency of the oscillation signal K depends on the voltage V, as mentioned earlier; the phase of the oscillation signal K is detected and represented by the phase sample P; the digital output D, which is a time-derivative of the phase sample P, represents the frequency of the oscillation signal K (because the frequency is a time derivative of the phase) and thus indirectly represents the voltage V. Detailed embodiments of V2I 211, ICRO 212, ring sampler 221, and phase decoder 222 can be found in the following two papers authored by G. Taylor, I. Galton: (1) “A Mostly-Digital Variable-Rate Continuous-Time Delta-Sigma Modulator ACD,” IEEE Journal of Solid-State Circuits, vol. 45, no. 12, pp. 2634-2646, December 2010; and (2) “A Reconfigurable Mostly-Digital Delta-Sigma ADC with a Worst-case FOM of 160 dB,” IEEE Journal of Solid-State Circuits, vol. 48, no. 4, pp. 983-995, February 2013.
An issue with ADC 200 is, in practice, the transfer characteristics of VCO 210 is not perfectly linear; that is, the frequency of the oscillation signal K does not change linearly with the voltage V. In this case, the digital output D does not accurately represent the voltage V, unless a calibration is performed. Although efforts have been made by G. Taylor and I. Galton to achieve accurate analog-to-digital conversion by calibration, the calibration scheme is cumbersome and takes a long time to finish.